This invention relates generally to radio frequency circuits and more particularly to radio frequency circuits which provide a selective phase shift to a signal propagating therethrough.
As is known in the art, phase shifter circuits are employed in various radio frequency (r.f.) applications such as phased array antenna systems. One type of phase shifter, a passive phase shifter 10 is shown in FIG. 1 and includes passive elements which provide a phase lag and a phase lead network and includes a pair of signal paths provided between an input terminal RF IN and an output terminal RF OUT with the upper one of the signal paths being through a high pass filter 14 to provide phase lead or positive phase shift to a signal and the lower one of the signal paths being through a low pass filter 18 to provide phase lag or negative phase shift to a signal. Typically, a pair of switches 12, 16 are used to couple a signal between the input and output terminals through a selected one of said filter networks. Often, a pair of field effect transistors are arranged to provide active switching elements of each one of said switches. Field effect transistors are employed in these applications because they are easily formed as part of monolithic integrated circuits unlike other types of active switching devices such as pin diodes. A problem with the approach shown in FIG. 1 is that the input RF signal coupled between the input terminal and the output terminal first passes through the input switch 12 and thus a selected one of the input switching field effect transistors thereof, a first selected one of the high and low pass filter sections, and the output switch 16 and thus a selected one of the output switching field effect transistors before reaching the output terminal. That is, the input signal is cascaded through an input switching field effect transistor, a selected filter section and an output switching field effect transistor. Since each one of said field effect transistors has a finite bandwidth, the overall bandwidth of the circuit will be less than the intrinsic bandwidth of the filter section alone.
An additional problem with this approach is that in the "off state" field effect transistors generally have a very large drain-source capacitance which permits a portion of the input signal to be coupled between the input and output terminals through the "unselected" filter section. This generally causes phase distortion in the output signal. Generally, therefore, this large capacitance must be compensated for by tuning or resonating the capacitance. However, this solution generally reduces the bandwidth of the phase shifter from that of the theoretical bandwidth of the passive network.
Further, with this approach, impedance matching networks (not shown) are generally required at the input and output of each FET. This results in a loss in bandwidth, increased insertion loss and increased complexity and size of the phase shifter circuit.